Apparatus for lubricating pneumatic rappers

ABSTRACT

A pneumatically operated rapper including a piston reciprocable within a cylinder adapted to strike an anvil within said cylinder thereby imparting vibrations to a structure connected to said rapper, wherein the improvement comprises providing said piston and said cylinder of dissimilar materials of which one material possesses dry lubricating properties and one of said dissimilar materials is covered with a thin film of low friction material which is erodable by abrasion between said piston and said cylinder to form microscopic particles for coaction with said dry-lubricating material, thereby conditioning the coacting surfaces of the piston and cylinder for reduced friction and continuous dry-lubrication thereof.

United States Patent Primary ExaminerManuel A. Antonakas Attorneys-OscarB. Brumback, Boyce C. Dent and Olin E.

Williams ABSTRACT: A pneumatically operated rapper including a pistonreciprocable within a cylinder adapted to strike an anvil within saidcylinder thereby imparting vibrations to a structure connected to saidrapper, wherein the improvement comprises providing said piston and saidcylinder of dissimilar materials of which one material possesses drylubricating properties and one of said dissimilar materials is coveredwith a thin film of low friction material which is erodable by abrasionbetween said piston and said cylinder to form microscopic particles forcoaction with said dry-lubricating material, thereby conditioning thecoacting surfaces of the piston and cylinder for reduced friction andcontinuous dry-lubrication thereof.

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l INVENTOR.

\ mum zooms APPARATUS FOR LUBRICATING PNEUMATIC RAPPERS BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates generallyto gas separation devices and more particularly to vibrating, jarring,or rapping means for the collector and discharge electrodes of anelectrostatic precipitator.

2. Description of the Prior Art Conventionally, in electrostaticprecipitators, a particle laden gas stream is directed through banks oflarge vertically suspended sheet metal collector electrode plates.Between these collector plates are vertically suspended dischargeelectrode wires. High voltage applied to the electrode wires ionizes thegas. Dust particles in the gas become charged and are thus attracted tothe collector electrode plates. The plates and wires must be kept freeof accumulated dust to function properly. Pneumatic rappers are oftenused to transmit vibrations to the electrodes thus causing the depositeddust particles to break loose and fall into a hopper at the bottom ofthe precipitator. Rappers of both the single impulse type and therapidly reciprocating type are used. The precipitation operationdescribed in connection with the present invention is best accomplishedby the use of a rapper of the rapidly reciprocating type.

One recurring problem in such rappers is the lack of an adequatelubricating means between the coacting annular surfaces of the pistonand the rapper housing. Improved lubrication means would greatlyincrease the life of the rapper assembly, require a minimum of servicingand maintenance, provide the necessary durability to withstand the rapidreciprocations, and be produced economically.

One solution to the problem as shown in Penningtons US. Pat. No.3,030,753 is to apply a polytetrafluoroethylene (hereinafter referred toas PTFE) tape to the wall of the piston thus reducing friction betweencoacting annular surfaces of the piston and the rapper housing. Whilesuch use of the PTFE tape has improved the life of the rapper assembly,it has presented production problems because of the need to achieve afirm bond when applying the PTFE tape to the piston wall. Penningtonthreads the outer surface of the bearing lands and then applies an epoxyresin thereto to bond the tape to the lands, clamps the tape in placeand cures it in an oven. This method of production has proved to beexpensive. Also, it has been found thatthe tape must be at least threethirty-seconds of an inch in thickness to wear properly and if it isgreater-than one-fourth of an inch thick the leading edge tends to flowor stretch in operation. Such flowing or stretching sometimes causes thetape to be severed from the piston.

An improvement was provided by Rodgers et al. as disclosed in theiraforementioned application wherein it is disclosed, among other things,that a ductile iron piston disperses a sufficient amount of nodulargraphite to adequately lubricate the coacting annular surfaces of apiston and cylinder provided the piston reciprocates slowly. Thus, thisimprovement works best for single impulse rappers. It does not improvethe operation of rapidly reciprocating rappers as much as desired.

SUMMARY OF THE INVENTION It has been discovered according to the presentinvention that a lubricating means can be provided which performssatisfactorily in a rapidly reciprocating rapper by cooperating with thematerials comprising the piston and the rapper housing to achieve thedesired results. This means includes a housing made of a first materialand a piston made of a dry lubricating material having PTFE sprayed onthe annular surface of the piston.

Surprisingly, it was found that a synergistic action occurs between thePTFE and the coacting annular surfaces of the piston and the housing,that is, some of the PTFE erodes by abrasion of the coacting annularsurfaces thus producing microscopic particles of PTFE which thencooperate with the dry lubricating material also eroded from the pistonafter portions of the PTFE film have worn away. The particles conditionthe coacting annular surfaces thereby providing a continuous drylubricant between them for reducing friction and increasing wear life.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings wherein like parts aremarked alike:

FIG. i is an end view of the rapper assembly;

FIG. 2 is a cross-sectional elevation of the rapper assembly taken alongline II-ll of FIG. 1 including a partial cross-sectional elevation ofthe piston.

DESCRIPTION OF THE PREFERRED EMBODIMENT supporting structure (not shown)by a rod 48.

Spring 24 urges piston 30 toward the upper portion of cavity 16 at theend of each rapping cycle to prevent piston 30 from stopping in thecenter of the cavity 16 in a null position. The upper end of cavity 16is closed with end cap 20 by an interference fit between it and enlargedcavity 21. A snap ring 22 seated in a groove 23 provides a fail-safeconnection for the end cap 20.

Due to the rapid reciprocation of piston 30, usually between 3000 and4000 cycles per minute, rapper assembly 12 tends to be driven downwardinto tighter engagement with rod 48, however, since rapper 12 maybe usedin the horizontal position, bolts 46 and nuts 47 are provided to clampthe flanges 41 formed by slot 43 and stress relieving hole 65 tightlyagainst the tapered rod 43 seated in a corresponding tapered hole 49 inhousing 14.

When a piston reciprocates within a cylinder at between 3000 and 4000c.p.m., a serious lubrication problem is developed. A satisfactory meansof lubricating the coacting annular surfaces of the piston and thecylinder is needed to enable the rapper to function properly as it wouldnot be feasible to use a rapper which had to be continuously servicedand often replaced.

The use of oil as a lubricant is unsatisfactory since an elaborate meansof evenly distributing oil throughout the cylinder must be used and thesupply of oil must be constantly replenished.

It has been found that if the annular surface 33 of cavity 16 is made ofa first material having no particular lubricating properties and thecoacting annular surface 35 of piston 30 is made of a dry lubricatingmaterial such as ductile iron and a film of PTFE 37 is provided on oneof the annular surfaces, some of the PTFE 37 erodes from abrasionbetween the annular surfaces during reciprocation of the piston tocondition the surfaces for reduced friction operation and continuous drylubrication.

Although several material. combinations fall within the preferredboundary conditions hereinbefore stated it has been discovered thatcertain materials perform to a higher degree of proficiency than others.

Accordingly, the preferred material for the annular surface 33 of cavity16 is steel roll finished atieast as smooth as 400 R.M.S. or ground orturned to a finish at least as 200 EMS. and then roll finished tosubstantially 30 R.M.S. The annular surface 35 of piston 30 is a drylubricating material, preferable ductile iron which contains dispersednodules of graphite.

The PTFE film or coating 37 is preferably applied to the outer surface35 of the piston 30 rather than surface 33 of cavity '16. The outersurface of the piston rubs against the inner wall 33 of the cylinderduring reciprocation causing erosion of the PTFE coating. This erosioncauses microscopic particles of PTF E to form and causes the surface ofthe ductile iron to become gradually exposed. Upon exposure of theductile iron, nodules of graphite break free and form into microscopicparticles. These particles combine with the particles of PTFE to form aconditioning mixture of particles which tend to become imbedded in therubbing surfaces of the piston and cylinder.

it is believed that the use of ductile iron as the piston materialenhances the tendency of the powder mixture to become imbedded since theductile iron is inherently more porous than the steel surface 33.Nevertheless, some of the mixture becomes imbedded in surface 33 so thatthe end result is that the rubbing surfaces between the piston andcylinder are both coated with a low friction material. Of course, thesurface 33 can also be made of ductile iron but since the preferredcombination works well, there is no special need to do this.

The PTFE coating may be applied to either the annular surface of thepiston or cylinder and reach substantially the same results.

However, it is usually easier to coat the PTFE 37 onto the annularsurface 35 of the piston 30 than to the surface 33. As illustratedherein, coating 37 is sprayed on the annular surface 35 of piston 30 inlayers of between .0006 inches and .0012 inches thick. Additional layersare applied until the coating 37 is between .0006 and .0020 inchesthick. Advantageously, the thickness of coating 37 does not exceed amaximum of .0030 inches thick because the coating is expensive and acoating of greater thickness than .0030 inches does not erode at adesirable rate or expose a sufiicient surface area of piston 30.Coatings of a thickness less than .0006 inches on the other hand do notprovide enough PTFE particles upon erosion to create a sufficientmixture with the nodules of graphite to provide the desired lubricatingproperties. Coating 37 may be applied to piston 30 by conventionalmethods. However, spraying of the PTFE to the surface is convenient andprovides a uniform coating of PTFE.

In operation, air under pressure is supplied from a suitable source (notshown) and enters housing 14 through fitting 26 and is directed intoannular groove 28. Piston 30 is thereby activated due to the air ingroove 28 passing alternately to opposite ends of cavity 16 throughducts 32 and 34 provided in piston 30. The network of ducts 32 and 34 inpiston 30 and vents in housing 14 cause air to be supplied to oppositeends of cavity 16 thus changing air pressure therein and causing piston30 to reciprocate rapidly in the conventional manner.

As piston 30 reciprocates, the coating 37 acts to reduce frictionbetween annular surfaces 33 and 35; however, after many reciprocationcycles, the particles of PTFE coating 37 slowly erode by abrasionbetween surfaces 33 and 35 thereby producing microscopic particles ofPTFE. Gradually, portions of annular surface 35 of piston 30 becomeexposed thus causing nodules of graphite to be dispersed within cavity16.

When the nodules of graphite are dispersed, they mix with i themicroscopic particles of PTFE already eroded and form a dry lubricatingmixture within cavity 16. This mixture becomes imbedded in surfaces 33and 35 due to the rubbing of the surfaces thus creating a conditioningof the surfaces and greatly reducing friction between them. in addition,the mixture provides a continuous dry lubricant to further enhance thecoaction of the annular surfaces 33 and 35.

Tests indicate that the present invention provides continuouslubrication for pistons reciprocating rapidly within a rapper housingwhich requires no servicing or replacement for long periods of time.

Having thus described the invention in its best embodiment and mode ofoperation, that which is desired to be claimed by Letters Patent is:

lclaim: 1. In pneumatically operated apparatus including a pistonrapidly reciprocable within a cylindrical housing to impart vibrationsto a structure connected to said housing and wherein an outer annularsurface of said piston is in rubbing engagement with an inner annularsurface of said cylindrical housing, the improvement comprising:

one of said annular surfaces comprising a f rst material havingdry-lubricating particles therein and the other of said annular surfacescomprising a second material different from said first material; and

a substantially thin coating of PTFE provided on one of said annularsurfaces whereby the rubbing engagement resulting from reciprocation ofsaid piston forms microscopic particles of said PTFE from said coatingand releases microscopic particles of dry-lubricating particles fromsaid first material for the mixing with said particles of PTFE forconditioning said annular surfaces for reduced friction.

2. The apparatus of claim 1 wherein said first material is ductile ironhaving nodules of graphite therein.

3. The apparatus of claim 2 wherein said second material is steel.

4. The apparatus of claim 3 wherein said PTFE coating comprises acoating of PTFE having a thickness of between .0006 inches and .0030inches.

5. The apparatus of claim 4 wherein said inner annular surface has arolled finish no coarser than 400 R.M.S.

6. The apparatus of claim 4- wherein said inner annular surface has arolled finish at substantially 30 R.M.S.

7. The apparatus of claim 3 wherein said coating of PTFE is applied tosaid steel surface.

1. In pneumatically operated apparatus including a piston rapidlyreciprocable within a cylindrical housing to impart vibrations to astructure connected to said housing and wherein an outer annular surfaceof said piston is in rubbing engagement with an inner annular surface ofsaid cylindrical housing, the improvement comprising: one of saidannular surfaces comprising a first material having dry-lubricatingparticles therein and the other of said annular surfaces comprising asecond material different from said first material; and a substantiallythin coating of PTFE provided on one of said annular surfaces wherebythe rubbing engagement resulting from reciprocation of said piston formsmicroscopic particles of said PTFE from said coating and releasesmicroscopic particles of dry-lubricating particles from said firstmaterial for the mixing with said particles of PTFE for conditioningsaid annular surfaces for reduced friction.
 2. The apparatus of claim 1wherein said first material is ductile iron having nodules of graphitetherein.
 3. The apparatus of claim 2 wherein said second material issteel.
 4. The apparatus of claim 3 wherein said PTFE coating comprises acoating of PTFE having a thickness of between .0006 inches and .0030inches.
 5. The apparatus of claim 4 wherein said inner annular surfacehas a rolled finish no coarser than 400 R.M.S.
 6. The apparatus of claim4 wherein said inner annular surface has a rolled finish atsubstantially 30 R.M.S.
 7. The apparatus of claim 3 wherein said coatingof PTFE is applied to said steel surface.